One of the things President Reagan forgot to boast about in his State of the Union message was his gift to the fashion industry. What Nancy Reagan has done for red in women's clothes, Ronald Reagan has done for brown in men's.

You may not have noticed; it's happened slowly. Unlike the explosive phenomenon of Nancy Reagan and red, which in the first years of the administration had women grabbing any red dress in the store, the influence of her husband's brown suits has been gradual, percolating first through the fashion crowd.

Brown was a key color in the fall collections of Giorgio Armani and others shown earlier this month in Milan. Even Rei Kawakubo, the designer for Comme des Garc ons who has rarely strayed from blacks and grays, has started to include brown for menswear.

The designers aren't just remaking the Old Hollywood style the president prefers. Their version of brown is different. There is a lot of green and black in the brown President Reagan wears; the new brown is brown mixed with red and yellow, subtly striped or plaided with mauve, blue or even teal.

Brown has surfaced partially as a response to the overdose of black and the continual success of gray, says Armani. He hesitates to peg it to President Reagan specifically. "It is not a direct influence," says Armani. "But President Reagan does dress in a bit of a retro style. I take off on the elegance of that style where your president takes it seriously," he said.

Ralph Lauren has always used brown tweeds for men. He rarely wears brown himself -- "It just doesn't look good on me" -- though he started the whole idea of brown suede shoes worn with a blue striped suit. He tilts to a brown with red in it; an "English brown," he calls it. "Brown comes and goes and you always sell a little." Some find it old-fashioned, antique, "but for me that's good," says Lauren. "Now we're into a more vintage look, like an old Gary Cooper Technicolor movie."

Although Calvin Klein says "President Reagan always looks comfortable and elegant in his own special way," the designer does not believe one man can set trends in terms of color. "Navys and browns have always been men's colors," he says.

Bill Blass, who has made some of the best-looking clothes for Nancy Reagan, agrees. "Nancy made red her official color and turned others on to red," said Blass. "But the president ... Well, men do not play follow-the-leader in this instance."

Brian Bubb, the new designer of Perry Ellis menswear, says he's using a lot of brown for next fall, particularly a multicolor like a bark brown, a shetland brown and even a vicunåa. "I've loved brown and blue even when the rest of the world was pushing bright colors," the 30-year-old designer says proudly. "Brown was never supposed to be a power color, but maybe the president has changed that."

Chip Talbot thinks he has. Talbot, fashion director of the Men's Fashion Association, the educational and promotional arm of the men's fashion industry, ties the renewed interest in brown directly to an increased awareness of the president's penchant for brown suits. "We started to see a sign of it six years ago and it still continues," says Talbot. "Even John Molloy, author of the highly successful 'Dress for Success' books, now approves of brown," he laughs.

The new browns are compound colors, rather than flat single tones, he explains. The old brown was mixed with things like olive. "There is nothing more unattractive than olive next to the skin," insists Talbot. On the other hand, the new browns, mixed with red, turquoise and jade, for example, are extremely flattering. "Picture this brown suit from Alexander Julian with a teal overplaid. It's got six or more colors with the brown. It's much more desirable because it is much more flattering."

So far selling brown in Washington has not been easy. In fact, James Guerra, senior vice president for menswear for Raleighs and Garfinckels, thinks the talk about brown is more talk than reality. "It's a tough sell; most men prefer navy and gray." Guerra thinks there is almost a stigma about brown. "When I listened to a focus group in San Francisco while I was at Neiman-Marcus, I heard men say they would not be caught dead after 5 with a brown suit," says Guerra. "Most men look great in tuxedos because black is so flattering. Brown isn't."

At Britches, says President Richard Hindin, "Brown is not, has not and will not be an important color for executives." "Below the threshold of consciousness, where human reactions take place," he says, "navy blue and dark gray evoke leadership, power of character, sincerity. And our customers, who are primarily executive, need those emotional reactions. Brown doesn't do it. Brown says 'boring.' "

When several of his 1,500 students from the Pentagon, whom he coaches in preparing to move from a lifetime of uniforms into the color-conscious world of civilian careers, recently asked Hindin why the president can wear brown and they can't, Hindin replied, "He's got a job and he's not running again."

But Gene Pressman, executive vice president of Barney's in New York, says men with style wear "whatever they think looks good, brown or otherwise."

And maybe, just maybe, President Reagan has made them feel that way.